1. Field of the Invention
The present invention relates generally to transmission systems, and more specifically to a network node for an asynchronous transfer mode (ATM) transmission system for efficient utilization of virtual channels.
2. Description of the Related Art
FIG. 1 illustrates a prior art multiplex transmission system for an ATM network. In this prior art system a network node is comprised of a main ATM switch 10, an auxiliary ATM switch 11 and a plurality of multiplexer/demultiplexer units 12 and 13. Mux/demux units 12 and 13 are connected by respective two-way transmission lines to distant network nodes 14 and 15. In each of the mux/demux units 12,13, a plurality of virtual channel identifiers (VCIs) are respectively mapped to the identifiers of user cells which are switched through the main switch 10 in both directions of transmission. In the outgoing transmission, the VCI value of outgoing user cells from the ATM switch 10 are translated in the header translation and switching unit 11 where the cells are switched through permanent virtual connections 16 and 17 to the mux/demux units 12, 13. Each mux/demux unit uses the mapped relationships to multiplex the outgoing user cells into an outgoing cell of the corresponding VCI for transmission to the respective distant node. In the incoming direction, the process is reversed so that a plurality of user data in an incoming cell of a given VCI value are demultiplexed into a plurality of user cells of the given VCI value. The VCI value of these incoming user cells is translated in the auxiliary switch 11, where the demultiplexed incoming user cells are switched through the permanent virtual connections 16 and 17 to the main switch 10.
However, all virtual channels provided by the mux/demux units 12 and 13 are fixed and cannot be altered according to varying traffic between the nodes. The mux/demux units are standardized according to different patterns of inter-node traffic so that the number of virtual channels that can be provided by each standardized unit is varied in increments of a predetermined number. Therefore, if the number of virtual channels (or VCIs) actually required for each transmission link is smaller than the maximum number, unused virtual channels represent a significant wastage of network resource. Further, because of the fixed allocation of virtual channels, unused virtual channels of one transmission link cannot be allocated to other links.